1. Field of the Invention
The present invention relates to a control of an automatic transmission of a vehicle, and more particularly, to a speed stage shifting of an automatic transmission of a vehicle which is carried out by a change-over of engagement and disengagement of friction engaging means.
2. Description of the Prior Art
In an automatic transmission constructed as a tandem combination of a first gear unit including a first planetary gear mechanism and friction engaging means for changing over the speed change gear ratios thereof and a second gear unit including a second or a second and a third planetary gear unit and friction engaging means for changing over the speed change gear ratios thereof, so as to provide an overall speed change gear ratio by the multiplication of the speed change gear ratio of the first gear unit and that of the second gear unit, when the speed stage of the transmission is changed over in such a manner that the first and second gear units are shifted in opposite directions so that the first gear unit is shifted down whereas the second gear unit is shifted up, or vice versa, if the timing of the switching-over of the gear ratio of the first gear unit is not properly coordinated with that of the second gear unit, there would occur a steep change of the overall transmission gear ratio during the speed stage shifting, thereby generating a very uncomfortable speed stage shift shock in the vehicle. In more detail, when, for example, the first gear unit is shifted down by the disengagement of a first friction engaging means, while the second gear unit is shifted up by the engagement of a second friction engaging means, so as thereby to decrease the overall reduction gear ratio of the transmission, i.e. to shift up the transmission, if the downshifting of the first gear unit by the disengagement of the first friction engaging means starts virtually earlier than the upshifting of the second gear unit by the engagement of the second friction engaging means when the first and second gear units are instructed at the same time to start the respective shiftings, at an initial phase of the upshifting, the transmission gear ratio increases, contrary to an upshifting operation of the transmission. In order to solve this problem, it has been proposed in Japanese Patent Laid-open Publication 61-99745 to first start only the second friction engaging means of the second gear unit, and upon the detection of a virtual upshifting of the second gear unit, to instruct the downshifting of the first gear unit.
Further, also during the parallel progress of the upshifting of the second gear unit and the downshifting of the first gear unit, if the downshifting of the first gear unit advances relative to the upshifting of the second gear unit, there would also occur an undue shifting of the overall gear ratio toward the low gear side, thereby also deteriorating driving comfortableness of the vehicle. In view of this problem, it has been proposed in Japanese Patent Laid-open Publication 64-15560 to control the hydraulic pressure of either or both of the first and second friction engaging means by the art of feedback control so that the exhausting of the hydraulic pressure from the first friction engaging means is completed at the same time as the supply of the hydraulic pressure to the second friction engaging means. In this prior art, at an initial phase of the upshifting, the upshifting of the second friction engaging means is carried out alone by the feedback control so as to control the rate of progress of the upshifting of the second gear unit with a reference target value concerned with the second gear unit, and after the start of the downshift feedback control of the first gear unit, the feedback control of the second gear unit is only continued, so that two feedback controls are carried out in parallel.
However, the performance of the feedback control of the transmission at the phase where only the second gear unit is shifted up is largely different from that of the feedback control of the transmission at the phase where the second gear unit is shifted up while the first gear unit is shifted down, due to a change of the mechanical properties of the transmission. In this case, it is difficult to optimize the performance of the feedback control of the second gear unit throughout said first and second phases of the shifting.